And THAT’S why I drink…
I always thought one of O’Briens best moment was in DS9 season 1. He was trying to make a simple repair and triggered a computer lock down of the entire space station. 
Everything O’Brien tried just escalated the lock down protocols. They had to send out a rescue call to the Cardassians.
Talk about an embarrassing beginning in his new job. Oops.
I hated the catsuit, but loved the dress. She didn’t wear the dress enough.
I liked it okay, but the asymmetrical neckline always looked really weird to me.
I’m a clavicle man. She’s showing off just the right body parts for me.
The more she showed off back then, the better.
I thought the Captain “fell down the ladder” according to Hornblower, who we are led to believe, pushed him.
The senior officers, including Hornblower, were planning a mutiny because the doctor refused to relieve the increasingly insane captain of his command. It’s left ambiguous whether the captain’s fall was the result of this plot or just a happy coincidence which obviated the need for it. Regardless, it was only after the captain’s fall that the doctor was finally persuaded to keep the captain off-duty (even after he regained consciousness).
IIRC, Captain Sawyer spent a good amount of time laid up in bed doped up on laudanum before the discussion even came up. I can’t recall how it played out in the book, but I know on the TV series, the doctor only relieved the Captain of duty when he held Lieutenant Hornblower at gunpoint while making (incidentally accurate) mad declarations that Hornblower was plotting to undermine him.
The movie leaves the whole issue vague. The book manages to do so even further. In both cases, all of the likely witnesses to what happened end up either dead or Hornblower by the end of the story and Lieutenant Bush evidently gives up on any dream of getting a straight answer from Hornblower.
In the TV series at least, the doctor does get grilled pretty heavily over his decision to relieve the Captain. It looks like the Admirals are ready to assume he was wrong to do so right up until the matter of the Captain holding one of his own officers at gunpoint came up.
I thought Hornblower did it on his own.
It was at any rate a chance for Hornblower to become withdrawn, morose and seem all knowing, something he maintained for the rest of his life. 
Can’t let a dramatic angsty opportunity like that go to waste!
Well, withdrawn, morose, all-knowing, and dancing around naked on the deck of the ship while the crew sprayed him with sea-water.
Of course, that’s one of Hornblower’s quirks in the books too, which started in Lieutenant, actually, and continued onward in the later books.
Actually it was Bashir who was impersonated by a Changeling, but yeah. The creators called those their “Let’s Torture O’Brien” episodes.
Rank aside, it was pretty unbelievable that Troi was given command of the bridge over O’Brien in “Disaster”. Troi didn’t know what a warp core breach was! Yes, I know the dialogue–
O’Brien: …and we’ll have a containment breach.
Troi: Which means?
Ro: Which means the ship will explode!
–was written to convey this information to the audience. But it didn’t do much to improve my opinion of Troi’s command competence. Hell, even SHE was embarrassed enough to do something about it, taking the command-officer training a couple years later.
Actually, that should have improved your opinion of her. She recognized her own shortcomings, and took steps to correct them.
Even so, I think I’d rather see Miles in the command chair than Deanna.
O’Brien was also impersonated by a Changeling, at least briefly. He turns up on Earth just to mock Sisko over all the damage, confusion, paranoia, etc. that they had caused with only four Changeling infiltrators on the entire planet. One of those fun chances to see Colm Meaney twirl a villain mustache.
Yeah, looks like I conflated the O’Brien and Bashir impersonations. Anyway, my list upthread is probably only a small fraction of the terrible maladies to have befallen O’Brien. The misfortune even extended to the mirror universe, where his counterpart was a lowly slave who (according to post-DS9 licensed works) was murdered by his own son.
That’s certainly one of the two possible conclusions the reader is invited to draw in Lieutenant Hornblower. For some the matter is settled in The Life and Times of Horatio Hornblower, an unauthorized mock biography by C. Northcote Parkinson. It reproduces a long-lost letter from Hornblower in which he confesses not only to pushing Captain Sawyer down the ladder, but also slitting his throat while the Spanish prisoners were trying to take over the ship.
Whoa!
Been a long time since I read that.
There was also some exact duplicate of O’Brien who he had to allow to have his life temporarily, and then had to watch die in front of his eyes, even though the clone had never done anything wrong.
Could have been worse. His copy could have grown a goatee, joined the Maquis, and jumped shows to steal a space ship.